In disk storage systems, sampling by a disk controller of readback signals representing stored data information requires synchronization of the controller's clock to data pulses communicated from the disk. In conventional disk storage subsystems, this synchronization is achieved using a voltage controlled oscillator (VCO) to continuously adjust the frequency. To provide constantly accurate synchronization, the frequency of the oscillator at a set voltage has to be stable over a wide temperature range. Furthermore, to improve the sampling capability of the controller, it is desirable to have a VCO that has a wide range of linear voltage vs. frequency relationship.
On the other hand, the present industrial trend of increasing storage device capacity through data banding and constant data density recording also requires an oscillator that has a wide linear range of frequency vs. voltage relationship. (See "Constant-Density Recording Comes Alive With New Chips", Electronic Design, Nov. 13, 1986, pp. 141-144).
However, both the frequency vs. voltage relationship and the temperature sensitivity of a semiconductor device are dictated by the characteristics of its constituents. Since the frequency vs. voltage relationship of semiconductor components follows the exponential and square laws, and since semiconductor components are sensitive to temperature changes, currently available semiconductor VCOs consequently can only operate within a narrow range of linear frequency vs. voltage relationship, and have a strong dependency on their operating temperatures.
In current manufacturing processes of disk storage devices, the above mentioned desired capabilities of VCOs are attained by laser trimming a VCO so that the center point of its linear frequency/voltage region coincides with the central operating frequency of the system. Laser trimming the VCOs, however, is a costly process.
It is an object of this invention to provide a voltage controlled oscillator which has high temperature stability.
It is also an object of this invention to provide a voltage controlled oscillator in which the voltage to frequency relationship remains linear over a wide frequency range.